1. Field of the Invention
The present invention relates to electrophotographic printers, and, more particularly, to fusers in color electrophotographic printers capable of printing color transparencies.
2. Description of the Related Art
In an electrophotographic (EP) printer, unfused toner particles are electrostatically attracted to the media to form an image. In order for the image to be fixed permanently, the media and toner must be fused. During fusing, by combination of high temperature and pressure, the toner is melted and forced to adhere to the media.
In printing color transparencies, the fusing requirement is more stringent than merely ensuring that the toner adheres to the media. With multiple layers of toner, more energy is required to fuse the toner than in fusing a single layer of toner for monochrome printing. Unfused toner is opaque, and becomes transparent only upon application of sufficient heat. For a black transparency, under fusing, with some toner remaining opaque, is not problematic. However, for a high quality color transparency, sufficient energy must be added to the media and toner such that the toner becomes transparent. The ability to mix colors and the ability to produce good quality transparencies depends on the ability to make the toner transparent, which requires that all of the toner be adequately fused and that the toner surface be smooth.
The trend in current printer technology has been to reduce standby power requirements and reduce warm up times for the fuser. For this reason, a belt fuser with a ceramic heater is highly desirable. Due to the low thermal mass of such a fuser, it has a very short warm up time, and no standby mode is required. Belt fusers with ceramic heaters consume less overall power than other types of fusers and have a lower initial cost. This type of fuser has been used in several monochromatic printing applications; however, this type of fuser has not been successfully implemented for a versatile color printing application. Implementation for printing color transparencies has presented particular difficulties. Color fusers have to fuse a much higher toner mass/area ratio. The higher coverage presents two challenges; first all the layers of toners must be adequately fused, and second the fused toner must release cleanly from the belt surface. Color prints, and especially transparencies, are more sensitive to print quality defects than monochrome prints. For color transparencies, the toner must be smooth and free of surface defects that can scatter light, making the image appear “dirty” or out of focus. Most color fusers, therefore, are compliant hot roll fusers, which are expensive and slow to warm up. In more recent designs, induction heaters have been used in belt fusers. With an induction heater, the belt and heater are expensive, increasing the overall printer cost.
A fuser must supply sufficient heat to adequately fuse the toner, and must also remain below the release temperature limit, which is the temperature above which offset causes the toner to adhere to the fuser belt. Exit geometry can be used to aid in peeling the media from the fuser belt; however, it is desirable to expand the operating window, that is the temperature range between the minimum temperature for adequate fusing and the temperature at which unacceptable offset occurs.
Controlling the velocity of the media through the machine is an important function for all fusers, especially in compact color EP printers where the available space is minimal. Media handoff between the media transport belt and fuser is critical. If the speed from the media transport belt is faster than the fuser speed, the media will bubble and the surface can scrub against non-functioning machine surfaces, smearing the toner. If the media transport belt speed is slower than the fuser speed, the image can be smeared either in the developer or fuser nip. In an idling belt type fuser, unique problems can occur as a result of the fact that the pressure roller drives the media, which in turn drives the fuser belt. As the ceramic heater adds energy to the pressure roller, the material thereof is caused to expand, and the outer diameter increases, increasing the speed of the media. To meet the desirable goals of quick first copy time, the pressure roller does not come to a steady state temperature before printing begins, and the paper velocity in the fuser nip can change significantly from the first page to subsequent pages after steady state temperature has been achieved. Minimizing the variation in velocity is desirable.
What is needed in the art is an electrophotographic printer with a fuser having a fast warm up time, which fuses color images sufficiently even for transparencies, and which minimizes velocity variation through the fuser during prolonged operation.